The first goal of this proposal is to produce improved transgenic mouse models of sickle cell disease (SCD). The processes leading to vaso- occlusion, painful crisis and other pathophysiological aspects of SCD are complex and poorly understood. Animal models may be valuable for in vivo studies of acute and chronic vaso-occlusive processes, and for testing drug or gene therapies. A transgenic line expressing human HbS will be crossed with mice whose alpha and beta globin genes are deleted, to generate offspring expressing up to 100% human HbS. These mice will be phenotypically characterized as new SCD models. The second goal is to identify novel genetic modifiers of sickle cell disease, using techniques for genetic mapping the mouse. While the primary genetic determinant of SCD is the beta6Val mutation, there is considerable variability in the phenotypic expression of the disease among SS homozygotes, which may be due in part to genetic modifiers. The identification of such genetic modifiers could suggest new strategies for treatment, and provide prognostic indicators for individual SCD patients. While it would be difficult to identify such genes through human genetic studies, recent developments in mouse genetics permit the rapid mapping of modifier genes. The SAD-1 transgenic SCD model first will be bred into different inbred backgrounds to identify two strains in which the same transgene has quantitatively different phenotypic effects. The number of modifier loci will be estimated from an analysis of parental, F1 and N2 backcross generations. To map the modifier loci, a large pool of N2 animals will phenotypically characterized, and genotyped using polymorphic PCR markers, spanning the genome. Candidate genes mapping near the predicted modifier loci will be examined for allelic differences between the two strains, to identify potential modifier genes. In addition to this global approach, the role of a predicted modifier of red cell-endothelial interactions, von Willebrand factor (vWF), will be tested through crosses between SAD-1 mice and inbred RIIIS/J mice, which express lowered levels of vWF.